As the method known heretofore for diagnosing a valve apparatus as to occurrence of abnormality, there may be mentioned a method according to which a current and/or voltage of an electric power required for driving a stem coupled directly to a valve body (also referred to as the valve element) of the valve apparatus is measured upon a routine inspection thereof for thereby detecting a load acting on the stem, wherein existence of abnormality in the valve as well as in a stem driving unit is detected by making use of load information obtained in this way.
As a typical example of such abnormality, there may be mentioned fluid leakage through valve seats (also referred to as the inter-seat leakage). In that case, occurrence or existence of the inter-seat leakage is determined on the basis of vibration as detected. To this end, such an arrangement has heretofore been adopted in which a vibration sensor is mounted on the stem at the top end thereof for detecting vibration due to fluid flow through a valve body interlocked to the stem. Alternatively, the vibration sensor may be disposed in physical contact with an outer wall of the valve casing.
However, the conventional method mentioned above is disadvantageous in that because a great distance usually intervenes between the vibration sensor and the valve seats which constitute a source of vibration or because the measurement of the vibration is performed rather indirectly at a location deviated considerably from the vibration source, not only is great difficulty encountered in detecting the vibration with desired accuracy but also utility for practical applications is very poor because of incapability of making available other information than that of leakage, providing thus no remarkable contribution to enhancement of operation performance of the valve apparatus.
Besides, because the vibration sensor is not mounted integrally with the stem, it is impossible to calibrate the vibration sensor at the time of fabrication of the stem. Consequently, every time vibration is detected and necessity for identifying the cause for the vibration arises, the plant or an associated part thereof has to be once shut down to apply a reference vibration to the valve apparatus for calibrating the vibration sensor in order to decide what is indicated by the vibration detected by the vibration sensor.
Further, it is observed that the existing system includes no means for obtaining various information through the medium of temperature of a fluid being processed. Furthermore, since the temperature sensor is not permanently installed, problems arise in respect to the calibration and the processing of the signal delivered from the temperature sensor as in the case of the vibration sensor. It should further be added that although the information can be evaluated individually, synthetic evaluation is rendered impractical or impossible.
Under the circumstances, there exists a great demand for an abnormality detection apparatus or system which is capable of not only detecting leakages through the valve seats and the gland/packing assembly but also processing the information as obtained to thereby control the driving unit on the basis of the information as processed without need for shutting down the plant operation for evaluation of the information so that leakage can be coped with the valve apparatus by itself without resorting to other measures or external facilities.